the issue with OneLoop is that currently it is a sort of "legacy"
function which does some D->4 limits that are not always
safe. The error messages that you encounter come from other FeynCalc
functions "complaining" about the behavior of OneLoop.

This is why I usually recommend to avoid it for the time being and use
TID instead:

P.S. I started rewriting/improving OneLoop earlier, but it is a
very big chunk of work that will take me several months to complete.
When it comes to tensor reduction, TID is much more flexible than
OneLoop and is also well tested.